How serious is North Korea's nuclear threat? A look at what experts say

North Korean inter-continental ballistic rocket Hwasong-14 being prepared before a test launch at an undisclosed location in North Korea on July 4, 2017. PHOTO: EPA

WASHINGTON (AFP) - US intelligence believes North Korea has now built a nuclear weapon small enough to fit onto a ballistic missile, making it a potent threat against neighbours and possibly the United States, the Washington Post reported on Tuesday.

The country's nuclear advances have proceeded much more quickly than expected, but experts say North Korea still needs significant technological gains in order to become a full-fledged nuclear threat.

Where does the North Korean nuclear capability stand?

Pyongyang has conducted five nuclear bomb tests, with the last one, on September 9, 2016, roughly the size of the nuclear bomb the United States dropped on Nagasaki in 1945: 20-30 kilotons. That device, North Korea said, was going to be miniaturised and "standardised" for use on its ballistic missiles.

This year it demonstrated an ability to launch an intercontinental ballistic missile (ICBM) in two tests. The most recent of them, on July 28, showed a missile with a theoretical range of 10,000 km, meaning it could hit much of the United States and Europe, including population centres like New York and Paris.

Is that enough to constitute an immediate threat?

Besides reliable missiles with accurate targeting technology, Pyongyang needs to make sure its bombs would survive a 25,800 kmph re-entry from high in the atmosphere on an ICBM. It is possible their warheads are currently robust enough to survive the slower speeds of a shorter range missile that could strike Japan, but unlikely with an ICBM.

According to Michael Elleman, of the International Institute for Strategic Studies, the re-entry vehicle on the July 28 test likely broke into pieces and disintegrated.

Siegfried Hecker, a Stanford University nuclear expert, said it could be another five years before North Korea has an adequately robust reentry vehicle.

"I don't believe they have sufficient missile or nuclear test experience to field a nuclear warhead that is sufficiently small, light and robust to survive an ICBM delivery," Hecker told AFP.

What other technical hurdles does North Korea face?

Hecker, who has visited North Korea several times to view its nuclear activities, said its weapons program is deeply constrained by its small supply of uranium and plutonium, especially plutonium, which is preferrable in an ICBM-mounted weapon. Combined, he said, its uranium and plutonium supplies are likely enough for 20-25 nuclear weapons.

But according to the Washington Post report, the US Defence Intelligence Agency believes the country already has up to 60 nuclear weapons in its stockpile.

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